This invention relates to a process for the isomerization of n-paraffins to isoparaffins, with the particular aim of improving the octane number of certain petroleum fractions and more particularly those containing normal hexanes and pentanes, as well as branched hexanes and pentanes (C.sub.5 /C.sub.6 fractions).
Existing processes for the isomerization of C.sub.5 /C.sub.6 hydrocarbons using platinum catalysts of the chlorinated alumina type with a high activity operate on a once through basis, or with partial recycling, following fractionation of the unconverted n-paraffins, or with a total recycling after passing onto systems of molecular sieves in the liquid phase.
Although the once through process is simple, it is ineffective in increasing the octane number. To obtain high octane numbers, it is necessary to recycle constituents having a low octane number, after passing either into separating columns (e.g. a deisohexanizer) or onto molecular sieves, in the liquid or vapour phase.
A known isomerization process using molecular sieves for the vapour phase separation of the unconverted n-paraffins integrates the molecular sieve stage with the reaction stage. This is the so-called total isomerization process (or TIP), e.g. described in U.S. Pat. No. 4,210,771. It combines the use of an isomerization reactor supplied by the mixture of the charge, a desorption effluent and hydrogen and the use of a separating section by adsorption of the n-paraffins on the molecular sieve, desorption being carried out by hydrogen stripping. In such a process, the reaction system cannot consist of a high activity chlorine-containing alumina stage, due to the risks of contamination by hydrochloric acid of the integrated molecular sieves. Use is then made of a catalyst system having lower performance characteristics and which is based on zeolite and which does not use chlorine. This leads to a product having an octane number lower by 1 to 2 points than that which would have been obtained with a chlorinated alumina-based catalyst.
Thus, it is known that the lower the isomerization temperature, the higher the conversion of n-paraffins into isoparaffins and moreover the better the conversion of low octane number C.sub.6 isomers (methyl pentanes) into higher octane number C.sub.6 isomers (dimethyl butanes). It is also known that the platinum-impregnated, chlorinated alumina-based catalyst makes it possible to perform the isomerization reaction at a lower temperature than more stable, unchlorinated zeolite-type catalysts.
It was therefore of particular interest to conceive a process able to combine a low temperature reaction system (to have the optimum once through octane number rise) and a system of recycling the low octane number constituents of a non-integrated or chlorine-resistant nature.
It is possible to consider conventional system using separating columns (deisopentanizer and deisohexanizer), because the separating columns, can be immunized against chlorine contamination. However, such systems require a large amount of equipment and consume large quantities of energy, so that they are expensive to operate. A system having a single separating column (the deisohexaner only) would be less expensive, but would not be able to convert all the normal pentane into ispentane and would not therefore make it possible to obtain the increases in the octane number of diagrams using recycling.
To avoid contamination by chlorine of the molecular sieves used for separation, it is possible to consider an unintegrated system having a stage of stabilizing the isomerization effluent before supplying it to the adsorption stage. This idea was proposed in the so-called "PENEX MOLEX" combined process, in which the C.sub.5 /C.sub.6 hydrocarbons are isomerized in a chlorinated alumina catalytic reaction, followed by adsorption on a liquid phase molecular sieve of the normal paraffins from the bottom of the stabilizer and at the bottom temperature.
The use of a molecular sieve in liquid phase adsorption and desorption is more difficult than in the vapour phase. Thus, the ratio of the quantities of adsorbed normal paraffins to the isoparaffin quantities present in the mobile phase clearly favours vapour phase operation.
Another obstacle to the use of high activity catalyst systems is their sensitivity to the contaminants of the charge, namely sulphur and water. The liquid charge and the hydrogen top-up must be freed from sulphur and dehydrated prior to introduction into the reaction system. In the present state of the art using chlorinated alumina-based catalyst systems, the charges are dried in pretreatment operations using molecular sieves.